Allow the default port to be more easily configured.

Docker images and other automation use cases often want to make Jenkins
use specific port. Today they do this via init.groovy.d with a
problematic sleep timeout, so allow them to control this value via
system property.

core/src/main/java/jenkins/model/Jenkins.java

@@ -566,7 +566,7 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
      * TCP slave agent port.
      * 0 for random, -1 to disable.
      */
-    private int slaveAgentPort =0;
+    private int slaveAgentPort = Integer.getInteger(Jenkins.class.getName()+".slaveAgentPort",0);
 
     /**
      * Whitespace-separated labels assigned to the master as a {@link Node}.